Multiphase dc-dc converter and method for controlling a multiphase dc-dc converter

ABSTRACT

A method for controlling a multiphase DC-DC converter with two or more phase circuits, each with a switch arranged to control an inductor current through an inductor, the phase circuit is arranged to generate a phase current contributing to a total current to be delivered to an output side of the multiphase DC-DC converter. The method includes switching two or more of the phase circuits in Boundary Conduction Mode to generate interleaved phase current pulses, with a period length and a nominal turn-on time period of the switch; and, in at least one of the two or more of the phase circuits being switched, and for successive phases, repeatedly adapting the turn-on time period for controlling the length of the pulses of the inductor current to minimise a difference from the period length.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to the field of electronic power converters, andin particular to a multiphase DC-DC converter and a method forcontrolling a multiphase DC-DC converter.

Description of the Related Art

FIG. 1 shows a prior art boost converter for unidirectional powertransfer from an input side with input voltage U_(E), input capacitanceC_(E) and input current I_(E) to an output side with output voltageU_(A), output capacitance C_(A) and output current I_(A), an inductor orchoke with an inductance value L_(S), carrying an inductor currentI_(L), a diode with diode current I_(D) and a switch between a bridgepoint and a common terminal, the voltage between the bridge point andthe common terminal being U_(S). The switch is a semiconductor switch,for example a MOSFET switch. The input capacitance filters the inductorcurrent, resulting in the input current being the mean value of theinductor current. The output capacitance filters the diode current,resulting in the output current being the mean value of the diodecurrent.

FIG. 1 also shows the relevant voltages and currents over time,illustrating the commonly known Continuous Conduction Mode (CCM) inwhich the inductor current remains above zero. In the DiscontinuousConduction Mode (DCM), not illustrated, in periods of time when theswitch is open (U_(S)(t) being essentially equal to U_(A)) the inductorcurrent drops to zero and remains at zero for a certain period of time,until the switch is closed again. Closing the switch clamps the voltageat the bridge point to zero and causes the inductor current to riseagain. In the Boundary Conduction Mode (BCM, also called Transitionmode), not illustrated, the goal is to close the switch when theinductor current has dropped to zero, thereby eliminating the period oftime in which it remains at zero. In order to control the circuit inboundary mode, it is necessary to measure the inductor current.

It is known to combine such converters in parallel, creating amultiphase DC-DC converter. The separate phases must be controlled togenerate and add interleaved current pulses at the output side, in orderto reduce the ripple in the resulting output current. In the CCM, thecurrent in the different inductors must also be controlled to evenlydistribute the current over the phases, and to avoid that the currentrises to arbitrarily large values. Depending on requirements on thedynamics of the current, corresponding current sensors in each phasemust have a correspondingly high bandwidth. Switching losses occur bothwhen turning on an turning off the switches.

U.S. Pat. No. 7,884,588 B2 discloses a DC-DC converter with two or morephases and addresses the issue of the phases having different electricalparameters: in this case, if each phase is operated in BCM, then theywill run with different switching frequencies, and a ripple currentresulting from the addition of all phases currents will vary strongly.The solution proposed is to determine the phase that has the lowestswitching frequency when operated in BCM, and to operate the remainingphases with the same frequency, and in DCM. Optionally, in order toreduce capacitive losses, U.S. Pat. No. 7,884,588 B2 proposes to delaypower switch turn-on times in each phase. This comes at the cost of anincreased input current ripple, since the phase shift between theindividual input currents is no longer optimal.

A multiphase DC-DC converter can be structured as in FIG. 2, with, forexample, two or more phase circuits generating interleaved currentpulses.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to create a multiphase DC-DCconverter and a method for controlling a multiphase DC-DC converter,which overcomes one or more of the disadvantages mentioned above.

These objects are achieved by a multiphase DC-DC converter and a methodfor controlling a multiphase DC-DC converter according to thecorresponding independent claims.

In the method for controlling a multiphase DC-DC converter, themultiphase DC-DC converter is arranged for exchanging electrical powerbetween an input side an output side, the multiphase DC-DC converterincluding two or more phase circuits,

-   each phase circuit including a switch and an inductor, the switch    being arranged to control an inductor current I_(L) through the    inductor,-   the phase circuit being arranged to generate a phase current    contributing to a total current to be delivered to an output side of    the multiphase DC-DC converter. The method comprises:    -   switching two or more of the phase circuits in Boundary        Conduction Mode (BCM) to generate interleaved phase current        pulses, with a period length T and a nominal turn-on time period        t_(on) of the switch;    -   in at least one of the two or more of the phase circuits being        switched, and for successive phases, repeatedly adapting the        turn-on time period t_(on) for controlling the length of the        pulses of the inductor current I_(L) to minimise a difference        from the period length T.

As a result, it is possible, by interleaving the phase current pulsesgenerated by the phase circuits, to reduce the ripple in the outputcurrent, and at the same time to operate as many of the phase circuitsas desired in Boundary Conduction Mode (BCM). Operation in BCM, asopposed to DCM operation, reduces switching losses and EMC disturbances.Furthermore, as opposed to CCM operation, it eliminates the issue of thecurrent rising to arbitrarily large values.

The period length T and the turn-on time period t_(on) for each phasecircuit can be adapted in order for the sum of the interleaved phasecurrents to conform to a required total current. The period length Tshould be the same for all phase circuits, since their current pulsesare to be interleaved. The turn-on time period t_(on) would be the sameif the phase circuits had identical electrical parameters. In reality,variations of these parameters, especially of inductances, will causethe phase circuits to have different change ratios in their phasecurrents. This again would cause them not to operate in BCM if they allwere operated with the same values for their turn-on time period t_(on).Controlling the turn-on time period ton individually for each phaseallows to perform BCM operation even if the initial or nominal turn-ontime period t_(on) does not cause the inductor current I_(L) to returnto exactly zero at after the period length T—as it theoretically should.

Adapting, in subsequent switching periods, the turn-on time period tonaccording to the deviation of the actual zero crossing time from thedesired zero crossing time (determined by the period length T)synchronises the phase circuits.

In embodiments, the control method includes determining the periodlength T by operating one of the phase circuits, called master phase,with a turn-on time period t_(on) that is determined according to a meancurrent to be delivered by this phase circuit, and operating one or moreof the remaining active phase circuits to adapt their timing and periodlength to that of the master phase. That is, the remaining active phasecircuits adapt their turn-on time periods t_(on) so that they achievethe period length T determined by the master phase. Alternatively, acontroller determines a prescribed period length T and an initial valuefor the turn-on time periods t_(on) according to a mean current to bedelivered by each active phase circuit, and operates all the activephase circuits to adapt their turn-on time periods t_(on) in order toachieve the prescribed period length T.

The period length T and turn-on time period t_(on) in each phase circuitcan thus be determined by calculation, given the mean current to bedelivered. Controlling the phase circuit with this period length T andturn-on time period t_(on) will ideally result in the mean current to bedelivered, without the need for a current measurement in the respectivephase. This eliminates the need for a current sensor that returnsquantitative current measurements, as opposed to a sensor that onlydetects whether a threshold has been crossed.

In reality, the actual mean current may not have the exact valueprescribed by the mean current to be delivered. However, this will leadto a control deviation in a signal that is affected from the meancurrent, and a controller for that signal can adjust the mean current tobe delivered accordingly. Typically, this can be done by the controllerhaving an integral part.

A supervisory control loop can input the total current to be deliverede.g. from a total current set point, corresponding to the required totalcurrent that is to be delivered by the multiphase DC-DC converter. Ifthe total current set point is not reached, the supervisory control loopcan adapt the total current to be delivered. Thereby deviations of theactual parameters of the phase circuits from the nominal values can becompensated for.

When operating a phase circuit in BCM, the respective switch is turnedon, thereby starting a current pulse, just after the inductor currentI_(L) has returned to zero, at a zero crossing time. There is nosubstantial time period in which the inductor current I_(L) is zero, aswould be the case in Discontinuous Conduction mode (DCM).

The number of phase circuits to be active can be chosen depending on arequired total current to be delivered to the output side, and on themaximum and minimum current that can be delivered by each phase circuit.The minimum current depends on the voltage ratio, the maximum allowedfrequency and the shortest possible turn-on time period t_(on). Themaximum current depends on the maximal current that can be carried bythe inductor, upper branch switching unit, in particular a diode of thebranch switching unit, and by the switch, with a safety margin, e.g.,for controllability. Within these bounds, the number of phase circuitsbeing active can be chosen. Furthermore, the number can be chosen forthe current per phase circuit to be in a range where the circuit'sefficiency, or another criterion, is optimal.

The exact method for determining the number of phase circuits to beactive is outside the scope of the present invention which addresses,among others, the issues of operating a given number of active phasecircuits, and changing the number of active phase circuits.

In embodiments, the multiphase DC-DC converter is arranged forexchanging electrical power between an input side, including a firstinput terminal and a second input terminal, and an output side,including a first output terminal and a second output terminal, themultiphase DC-DC converter including two or more phase circuits,

-   each phase circuit including an inductor connected between the first    input terminal and a bridge point, a upper branch switching unit    connected between the bridge point and the first output terminal,    and a switch with a parallel freewheeling diode connected between    the bridge point and the second input terminal and second output    terminal.

Therein, the control method comprises the steps of,

-   -   determining a number N of phase circuits to be active;    -   determining an initial value for a turn-on time period t_(on);    -   determining a period length T;

for at least one of the phase circuits that are to be active;

-   -   turning on the switch of the phase circuit, causing an inductor        current IL and the switch to increase over time, and determining        a target turn-on time when the switch shall be turned on the        next time;    -   after the turn-on time period t_(on), switching off the switch,        causing the inductor current I_(L) to flow through the upper        branch switching unit and decrease over time;    -   after the current through the inductor has returned to zero, at        a zero crossing time, turning the switch on again and repeating        the above steps;    -   when repeating the above steps, controlling the turn-on time        period t_(on) by increasing the turn-on time period t_(on) if        the zero crossing time is before the target turn-on time and by        decreasing the turn-on time period t_(on) if the zero crossing        time is after the target turn-on time.

In embodiments, the step of determining the initial value for theturn-on time period t_(on) includes computing the turn-on time periodt_(on) so that a mean current I_(mean) through the inductor withinductance L is equal to a given value. This value can be determined bya supervisory control loop, e.g., from a total current set point,corresponding to the required total current that is to be delivered bythe multiphase DC-DC converter. In subsequent operation of theconverter, the turn-on time period t_(on) is adapted. The initial valuecan be used for feedforward control. This can improve dynamicperformance when the required current changes.

In embodiments, the initial value for the turn-on time period ton of aphase circuit is computed as

$t_{on} = \frac{2 \cdot I_{mean} \cdot L}{U_{IN}}$

wherein U_(IN) is the voltage at the input side, L is the inductancevalue of the inductor and I_(mean) is the mean current to be deliveredby the phase circuit.

In this way, the mean current to be delivered can be approximatedwithout the need of actually having to measure the inductor current, oranother current, during operation of the converter by a quantitativemeasurement. If the real inductance (which is not known) conforms to theinductance value L of the inductor (which is a nominal value, stored inthe controller), then the real mean current will essentially match themean current to be delivered. If it does not, then the difference can beeliminated by a controller that prescribes the mean current to bedelivered.

In embodiments, in the active phase circuits, the respective periodlength T is computed as

$T = {t_{on}\frac{U_{OUT}}{U_{OUT} - U_{IN}}}$

wherein U_(OUT) is the voltage at the output side.

The resulting period length T is a function of the actual input andoutput values, which can be measured, and the turn-on time periodt_(on), which in turn is a function of the mean current to be delivered,taking into account that the target is to operate in BCM.

In embodiments, the step of determining the target turn-on time includescomputing the target turn-on time as being offset from a reference timeby the period length T divided by the number N of phase circuits thatare active.

The target turn-on time for the next period can be computed before orafter the switch is turned on, depending on circumstances. The referencetime can be determined by a master phase or by a controller thatdetermines a prescribed period length T, as described above.

In embodiments, the step of turning the switch on again includes one of

-   -   monitoring the voltage across the switch and turning on the        switch when the voltage across the switch is zero;    -   monitoring the voltage across the switch, and turning on the        switch a predetermined time delay after the voltage across the        switch falls under a predetermined threshold;    -   monitoring the inductor current I_(L) and turning on the switch        when the inductor current I_(L) becomes zero after having been        negative due to a reverse current through the upper branch        switching unit;    -   monitoring the current through the upper branch switching unit,        in particular a diode current, and turning on the switch when        this current becomes zero after having been negative due to a        reverse current through the upper branch switching unit.

This allows to determine a time for at least approximately zero voltageswitching using a qualitative signal, i.e. determining when a valuecrosses a threshold.

In embodiments, the inductor current I_(L) is measured by measuring amagnetic field of the inductor. This can be done, for example, by meansof a Hall element.

In embodiments, the step of turning the switch on again includes

-   -   when the inductor current I_(L) has returned to zero, letting        the current reverse its direction and continue to flow through        the inductor and the upper branch switching unit until the upper        branch switching unit is turned off and the inductor current        I_(L) commutates to the freewheeling diode;    -   turning on the switch.

This allows for zero voltage switching of the switch.

In embodiments, the upper branch switching unit is constituted by adiode, or includes a diode, and is turned off by a reverse charge in thediode having built up, and in wherein a reverse charge of the diode ischosen so that the reverse current through the diode is sufficient todischarge capacitances between the bridge point and the second inputterminal.

In embodiments, the upper branch switching unit is constituted by adiode, or comprises a diode. A diode acts as a passive switch. Itswitches on and off depending on the current flowing through it. In suchembodiments, the upper branch switching unit can be turned off by areverse charge in the diode having built up. The reverse charge of thediode can be chosen so that the reverse current through the diode issufficient to discharge the capacitances between the bridge point andthe second input terminal.

In embodiments, the upper branch switching unit is constituted by anactive switch, or includes an active switch. An active switch switcheson and off depending on the state of a control signal. In suchembodiments, the upper branch switching unit can be actively turned offwhen at a point in time at which the capacitances between the bridgepoint and the second input terminal have been discharged.

In embodiments, the voltage across the switch is at least approximatelyzero at the same time when the inductor current I_(L) after flowingthrough the freewheeling diode in its forward direction has reversed itsdirection, driven by the input voltage, and returned to zero. This allowfor zero current switching in addition to zero voltage switching.

In embodiments, for increasing the number N of phase circuits that areactive to N+1, given a total current set point, the method comprises:

-   -   in a pre-transition period of length Tpi, switching the switches        of the N phase circuits to turn on at turn-on times, relative to        this period, of 0, dTpi, 2·dTpi, 3·dTpi . . . (N−1)·dTpi where        dTpi=Tpi/N;    -   computing, for a post-transition period of length Tsi, target        turn-on times for the N+1 phase circuits, relative to this        period, as 0, dTsi, 2·dTsi, 3·dTsi . . . N·dTsi where        dTsi=Tsi/(N+1);    -   in a transition period, switching the switches of the N phase        circuits to turn on at the same turn-on times, relative to this        period, as in the pre-transition period;    -   in the transition period, for each of the N phase circuits,        setting the turn-on time periods t_(on) so that the current        returns to zero at the respective target turn-on time in the        post-transition period;    -   in the transition period, after the turn-on time at (N−1)·dTpi,        turn on the switch of the newly operated (N+1)th at a time and        with e turn-on time period ton such that a deviation of the        total current of all phase circuits in the transition period and        the post-transition period from the total current set point is        (at least approximately) minimised.

This allows to include an additional phase circuit in the operation ofthe multiphase DC-DC converter, with a minimum effect on the quality ofthe total current. This in turn can be part of adapting to a loadchange: if a higher total is required, an additional phase circuit canbe activated in this way, which first reduces the mean current deliveredby each phase circuit. Subsequently, the mean current in each phasecircuit can be increased, thereby increasing the total current.

In embodiments, for decreasing the number N of phase circuits that areactive to N−1, given a total current set point, the method includes:

-   -   in a pre-transition period of length Tpd, switching the switches        of the N phase circuits to turn on at turn-on times, relative to        this period, of 0, dTpd, 2·dTpd, 3·dTpd (N−1)·dTpd where        dTpd=Tpd/N;    -   computing, for a post-transition period of length Tsd, target        turn-on times for the N−1 phase circuits, relative to this        period, as 0, dTsd, 2·dTsd, 3·dTsd . . . (N−2)·dTsd where        dTsd=Tsd/(N−1);    -   in a transition period, switching the switches of the N phase        circuits to turn on at the same turn-on times, relative to this        period, as in the pre-transition period;    -   in the transition period, for each of the N phase circuits,        except for the phase circuit whose target turn on time relative        to this period is dTpd, setting the turn-on time periods t_(on)        so that the current returns to zero at the respective target        turn-on time in the post-transition period;    -   in the transition period, for the phase circuit whose target        turn on time relative to this period is dTpd, setting the        turn-on time period t_(on) for one last pulse of this phase        circuit such that a deviation of the total current of all phase        circuits in the transition period and the post-transition period        from the total current set point is (at least approximately)        minimised.

This allows to remove a phase circuit in the operation of the multiphaseDC-DC converter, with a minimum effect on the quality of the totalcurrent. In analogy to in the above, this can be part of adapting to aload change, for providing a lower total current: first, the meancurrent in each active phase circuit is reduced, then one of the phasecircuits is deactivated, which increases the mean current in each of theremaining phase circuits.

In embodiments, for a transition between operation of the multiphaseDC-DC converter in discontinuous conduction mode to boundary conductionmode, for one or more pairs of phase circuits, the method comprises:

-   -   operating the respective two phase circuits of that pair to        generate alternating current pulses of the same shape, each of        these two phase circuits generating pulses and periods with zero        current of equal length, the length being the period length T;    -   switching one of the respective two phase circuits off and        generating current pulses with the period length T in boundary        conduction mode by the other one of the two phase circuits.

This allows to switch from DCM to BCM, thereby reducing the number ofactive phase circuits by a factor of two. The same principle can beapplied—if the number of phase circuits is large enough—for a factor ofthree or more.

In embodiments, the method includes, for a transition between operationof the multiphase DC-DC converter in discontinuous conduction mode toboundary conduction mode, for one or more sets of np phase circuitseach, np being two or more, for each of these sets

-   -   operating the respective np phase circuits of the set in        discontinuous conduction mode to generate a sequence of np        pairwise adjacent current pulses of period length T, each phase        circuit contributing one of the current pulses of the sequence;    -   not operating the respective np phase circuits of the set in        discontinuous conduction mode, and instead operating one phase        circuit in boundary conduction mode to continue the sequence of        current pulses of period length T.

The phase circuit that finally operates in boundary condition mode canbe one of the phase circuits that first operated in discontinuousconduction mode, or another one.

Several such sequences, each generated by one such set of np phasecircuits, can be interleaved.

In embodiments, for a transition between operation of the multiphaseDC-DC converter in boundary conduction mode to discontinuous conductionmode, for one or more pairs of phase circuits, the method comprises:

-   -   not operating one of the respective two phase circuits and        generating current pulses with the period length T in boundary        conduction mode by the other one of the two phase circuits;    -   at a point in time when the current is zero, switching to        discontinuous conduction mode by operating both of the        respective two phase circuits of that pair to generate        alternating current pulses of the same shape, each of these two        phase circuits generating pulses and periods with zero current        of equal length, the length being the period length T.

This allows to switch from BCM to DCM, thereby increasing the number ofactive phase circuits by a factor of two. The same principle can beapplied—if the number of phase circuits is large enough—for a factor ofthree or more.

In embodiments, the method includes, for a transition between operationof the multiphase DC-DC converter in boundary conduction mode todiscontinuous conduction mode, for one or more sets of np phase circuitseach, np being two or more, for each of these sets

-   -   operating one phase circuit in boundary conduction mode to        generate a sequence of current pulses with period length T in        boundary conduction mode;    -   not operating this one phase circuit in boundary conduction        mode, and instead operating the respective np phase circuits of        the set to continue the sequence of current pulses of period        length T by operating the np phase circuits in discontinuous        conduction mode to generate a sequence of np pairwise adjacent        current pulses of period length T, each phase circuit        contributing one of the current pulses of the sequence.

The phase circuit that first operates in boundary condition mode can beone of the phase circuits that then operate in discontinuous conductionmode, or another one.

The multiphase DC-DC converter comprises a controller including voltagesensors arranged for determining the voltage U_(IN) at the input side,the voltage U_(OUT) at the output side and the voltage U_(L) across theinductor of each phase circuit, the controller being configured toperform the method described herein.

In embodiments, the multiphase DC-DC converter and the controller arefree from a measurement of a current through the inductor and/or theswitch of the respective phase circuits.

In embodiments, in the multiphase DC-DC converter at least one of thephase circuits the upper branch switching unit includes or consists of adiode that includes a reverse recovery time that is sufficiently largeto reverse the inductor current I_(L) after the inductor current I_(L)has returned to zero, such that the reversed current discharges acapacitance of the switch and of a freewheeling diode and of a parallelcapacitance, if present, before turning on the switch.

This allows for zero voltage switching of the switch even in regard ofcapacitances inherent in the switch and/or freewheeling diode, and moregenerally of capacitances that can be added in parallel to the switch,for example to reduce switching losses.

In embodiments, the reverse recovery time is sufficiently large for thereversed current to also discharge a capacitor arranged parallel to theswitch before turning on the switch.

Further embodiments are evident from the dependent patent claims.Features of the method claims may be combined with features of thedevice claims and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail inthe following text with reference to exemplary embodiments which areillustrated in the attached drawings, which schematically show:

FIG. 1 a prior art DC-DC boost converter and typical values in CCMoperation;

FIG. 2 a multiphase DC-DC converter;

FIG. 3 the interleaving of current pulses;

FIG. 4 different trajectories of an inductor current I_(L);

FIG. 5 pertinent points in time when controlling the multiphase DC-DCconverter;

FIG. 6 voltage and current trajectories for a switch of a phase circuit;

FIG. 7 activation of an additional phase circuit and insertion of itscurrent pulses into a sequence of interleaved pulses;

FIG. 8 deactivation of a phase circuit and removal of its current pulsesfrom a sequence of interleaved pulses;

FIG. 9 switching between DCM and BCM and vice versa; and

FIG. 10 a method for controlling the converter in DCM.

DETAILED DESCRIPTION OF THE INVENTION

In principle, identical parts are provided with the same referencesymbols in the figures.

FIG. 2 shows a multiphase DC-DC converter 10. It includes, at an inputside, a first input terminal 11 and second input terminal 12, with aninput capacitance 13 arranged between the two input terminals. At anoutput side, it includes a first output terminal 14 and a second outputterminal 15, with a output capacitance 16 arranged between the twooutput terminals. Two or more phase circuits 20 are arranged to connectthe input and output terminals. In the present embodiment, the phasecircuits 20 are boost converters, each with an inductor 21 arrangedbetween the first input terminal 11 and a bridge point 22, an upperbranch switching unit 23 constituted by a diode arranged between thebridge point 22 and the first output terminal 14, and a switch 24 with afreewheeling diode 25 arranged between the bridge point 22 and thesecond input terminal 12 and second output terminal 15. The switch 24typically is a semiconductor switch, for example, a MOSFET, SiC, IGBT,GaN or other known switch type.

The figures show the upper branch switching unit 23 constituted by adiode, in other words, the upper branch switching unit 23 essentially isa diode. In other embodiments, the upper branch switching unit 23includes variants such as an active switch in parallel with a diode(shown in the rightmost phase circuit 20 of FIG. 2 in an exemplaryfashion, in reality, all phase circuits 20 typically have upper branchswitching units 23 of the same kind), or as an active switch alone (notshown).

The expression “arranged between” means that the respective elementconnects two points in the circuit, and can carry a current between thetwo points, depending on the state of the element.

A controller 40 is arranged to control switching of the switches 24 andto measure, e.g., voltages and currents in the multiphase DC-DCconverter 10, using sensors not shown in the figure. The controller 40can be configured to control a current delivered to the output side tofollow a total current set point. Such a set point can be determined bya supervisory control loop, depending on the circumstances under whichthe multiphase DC-DC converter 10 is operated.

The total current delivered to the output side is the sum of phasecurrents delivered by the phase circuits 20. The phase circuits 20 areoperated to generate interleaved current pulses, in order to minimise aripple in the total current. Depending on the required total current,according to the total current set point, the controller 40 candetermine an optimal number of phase circuits 20 to be active, so thatthe total current is delivered while each of the phase circuits 20operates in an optimal or near optimal condition, for example withregard to switching losses.

FIG. 3 shows the interleaving of current pulses with two phase circuits20 being active. The top graph shows the inductor currents I_(L) of thetwo phase circuits 20 being operated in BCM. In each of the successivecurrent pulses, the inductor current I_(L) rises to a positive peakvalue +I_(peak) while the respective switch 24 is closed, driven by theinput voltage across the inductor 21 and flowing through the switch 24.While the switch 24 is open, the inductor current I_(L) decreases again,driven by the difference between the output and input voltage across theinductor 21. While the switch 24 is open, the inductor current I_(L)flows through the upper branch switching unit 23 and constitutes thephase current delivered to the output side. The decreasing inductorcurrent I_(L) is allowed to go below zero, to a negative peak value−I_(peak) before the switch 24 is closed again. This allows for zerovoltage and/or zero current switching of the switch 24, as is explainedfurther below.

The bottom graph shows the output current I_(A) resulting from theaddition of the two phase currents. This output current I_(A) isfiltered by the output capacitance 16. Its mean current I_(Amean) can becontrolled according to the total current set point. The input currentI_(E) with mean value I_(Emean) is also shown. It is evident that for aswitching frequency corresponding to a switching time period T_(SW) ineach phase circuit 20, the output current has a period of T_(SW)/2 andthe input current has a period of T_(SW)/2 of the variation about itsrespective mean, corresponding to its respective ripple frequency. Theripple frequencies increase according to the number of active phasecircuits 20.

FIG. 4 shows, for a phase circuit 20, different trajectories over timeof an inductor current I_(L), together with a state SW of thecorresponding switch 24. When SW is high, the switch 24 is closed, i.e.,in a conducting state. When SW is low, the switch 24 is open, i.e., in anon-conducting state. Given a period length T (corresponding to aswitching frequency) between a turn-on time 31 and a next turn-on time31′, a turn-on time period t_(on) during which the switch 24 is closedis to be chosen such that the phase circuit 20 operates in BCM. This isillustrated by the BCM mode trajectory 38 in which the returns to zeroat the next turn-on time 31′.

The turn-on time period t_(on) and the period length T are calculatedsuch that a mean current to be delivered is generated and the currentwill return to zero at the end of the period length T. This requiresonly knowledge of the inductance value of the inductor 21, and isfurther determined by the input voltage and the mean current to bedelivered by the phase circuit 20.

In more detail, the turn-on time period t_(on) of a phase circuit 20 canbe computed as

$t_{on} = \frac{2 \cdot I_{mean} \cdot L}{U_{IN}}$

wherein

-   -   U_(IN) is the voltage at the input side, which can be measured;    -   L is the inductance value of the inductor 21, which can be        known, by the design of the inductor 21, or by measurements; and    -   I_(mean) is the mean current to be delivered by the phase        circuit 20. which can be given, e.g., by the controller 40.

The period length T can be computed as

$T = {t_{on}\frac{U_{OUT}}{U_{OUT} - U_{IN}}}$

wherein U_(OUT) is the voltage at the output side.

If two or more phase circuits 20 are to operate synchronously, with aphase shift of their respective phase currents according to the numberof active phase circuits 20, then their period lengths T should be thesame. The period length T to be used for all phase circuit 20 can bedetermined by different approaches:

In an embodiment, one phase circuit 20 is designated as Master, and theothers as Slaves. The Master is operated to run in “self-synchronised”mode. That is, the ideal period length T, computed as shown above is notreached exactly, but is determined by the actual time at which theinductor current I_(L) reaches zero.

The period length T determined in this manner by the Master is then usedfor the Slave phase circuits 20.

In another embodiment, the controller 40 determines the period length Tfor all the active phase circuits 20 together. This implies prescribinga mean output current for each of the phase circuits 20 according to thetotal current set point, and adapting the period length T and turn-ontime periods t_(on) for the phase circuits 20 accordingly.

In each of the different approaches, for any phase circuit 20 that isnot operated in “self-synchronised” mode, there is the issue ofoperating it in BCM with a prescribed period length T:

-   -   With the turn-on time period t_(on) computed as shown above,        ideally, the BCM mode trajectory 38 should result. In reality,        parameters of the phase circuit 20, in particular the inductance        value, will not be perfectly correct, or will drift over time.

If the real inductance value is lower than expected, or due to otherdeviations, a CCM mode trajectory 37 will be realised: while the switch24 is closed, the inductor current I_(L) will rise more than expected.At the end of the period length T, at the next turn-on time 31′, theswitch 24 is turned on again before the inductor current IL has returnedto zero. Over several periods, the inductor current I_(L) will keeprising. This is not acceptable.

If the real inductance value is higher than expected, or due to otherdeviations, a DCM mode trajectory 39 will be realised: while the switch24 is closed, the inductor current I_(L) will rise less than expected.At the end of the period length T, at the next turn-on time 31′, theswitch 24 is turned on again after the inductor current I_(L) hasreturned to zero. Depending on the corresponding delay, electromagneticdisturbances and switching losses will arise.

In order to keep avoid the phase circuit 20 operating in either CCM orDCM, and keep it in BCM, the period t_(on) is controlled: rather thanswitching the switch 24 on at the predetermined next turn-on time 31′,it is switched on as for self-synchronised operation, that is, after theinductor current I_(L) has returned to zero. The time at which it isswitched on is compared to the predetermined next turn-on time 31′, andthe turn-on time period t_(on) is adapted according to the different intime. A controller such as a PID controller can be used, and moregenerally, a controller that brings a steady state error to zero.Consequently, over a sequence of periods, the turn-on time period t_(on)is adapted so that the inductor current I_(L) returns to zero at thedesired respective next turn-on time 31′, corresponding to the desiredor predetermined period length T.

The period length T in turn can be adapted or varied by another, outercontrol loop, in order for the mean current of the phase circuit 20 andthe total current of the multiphase DC-DC converter 10 to follow avariation in their corresponding set points.

If the total current actually delivered by the multiphase DC-DCconverter 10 is not as it ideally should be, according to the aboveformulae, then the outer control loop can adapt the total current andthereby the mean current to be delivered by each phase. This will inturn cause the turn-on time period t_(on) and the period length T to beadapted.

FIG. 5 shows pertinent points in time when controlling the multiphaseDC-DC converter: inductor current pulses from one phase circuit 20,which can be a Master, are shown with drawn out lines, parts of otherpulses, which can be Slaves, are shown with dashed lines. The beginningof a pulse in the Master is used as reference time 35 (T_(ref)) for theother pulses. For the n-th Slave, a target turn-on time 34(T_(target)(n)) is determined as T_(target)(n)−(T·n)/N. In operation ofeach phase circuit 20, the respective actual turn-on time 36 can deviatefrom the target turn-on time 34, and the controller as described abovemodifies, for the next period, the turn-on time period t_(on).

If there is no Master phase and the period length T is prescribed forall active phase circuits 20, then all the active phase circuits 20 areoperated as Slaves.

The situation illustrated in the previous figures, and the calculationsfor determining the period length T and turn-on time period t_(on) areapproximations that do not consider the detailed current trajectory justbefore and after the turn-on time 31. This is acceptable since on theone hand the charges and currents involved in the switching operation,as explained in the context of FIG. 6, are much smaller than those overan entire period. On the other hand, small errors resulting from theapproximation will be corrected by the control of the turn-on timeperiod t_(on).

FIG. 6 shows a trajectory of the inductor current I_(L) and the voltageV_(S) across the switch 24 of a phase circuit 20. The voltage V_(S)across the switch 24 is the same as the voltage of the bridge point 22.At the turn-off time 32, the voltage rises and the current begins todecrease, flowing through the switch 24 and the upper branch switchingunit 23.

According to an embodiment, the switch 24 is turned on after theinductor current I_(L) has become zero: The inductor current I_(L),driven by the difference between the input and output voltages, becomesnegative to an extent depending on the time at which the upper branchswitching unit 23 is turned off. In the case in which the upper branchswitching unit 23 is constituted by a diode, this time depends on thereverse recovery charge of the diode. When the upper branch switchingunit 23 blocks the inductor current I_(L), it commutates to thefreewheeling diode 25 of the switch 24. The freewheeling diode 25becomes conducting and the voltage V_(S) across the switch drops tozero. When the voltage is zero, the switch 24 is switched on at turn-ontime 31. Ideally at this instant the inductor current I_(L) has returnedto zero again. As a result, the switch 24 is turned on at zero current,reducing EMC disturbances, and zero voltage, reducing switching losses.The figure shows, in addition to the inductor current I_(L) rising againafter the turn-on time 31, trajectories I′ and V′ that the current andvoltage would take if the switch 24 were not turned on.

Thus, the switching on can be triggered by a threshold detection of thevoltage V_(S) across the switch 24. The switching can be triggered whenthe voltage V_(S) is zero. Or the switching can be triggered apredetermined time delay T_(del) after the voltage V_(S) across theswitch 24 falls under a predetermined threshold V_(S_tresh) that islarger than zero. The predetermined time delay and threshold can bedetermined according to the parameters of the phase circuit, and storedin the controller 40. Triggering on the basis of the thresholdV_(S_tresh) that is larger than zero moves the point at which thethreshold is crossed to an earlier point in time and so allows tocompensate for processing time required by the controller 40.Alternatively, the switching on can be triggered by threshold detectionof the voltage at the bridge point 22, which usually is identical to thevoltage V_(S) across the switch 24.

Alternatively, the switching on can be triggered by threshold detectionof the inductor current I_(L). For this, inductor current I_(L) itselfcan be monitored, or the current through the upper branch switching unit23, since prior to commutation it is the same as the inductor currentI_(L). The inductor current I_(L) can be monitored by monitoring themagnetic field of the inductor 21.

In order for the current and voltage to be zero or near zero at the sametime, a diode constituting the upper branch switching unit 23 can bechosen to have a corresponding reverse recovery time. The reverserecovery time determines the time during which the inductor currentI_(L) is negative. The diode is chosen such that for nominal operationconditions the current and voltage are zero at the same time.

FIG. 7 shows activation of an additional phase circuit and insertion ofits current pulses into a sequence of interleaved pulses. Given a totalcurrent set point, in a pre-transition period the current pulses arehigher and the period length Tpi is longer than in a post-transitionperiod with period length Tsi. A transition period has the same periodlength Tpi as the pre-transition period and coincides with the pulseperiod of a phase that is designated as the Master. The other phasesalready active (Slaves) are operated to be

-   -   switched on in the transition period, with turn-on times        (relative to the period) as in the pre-transition period, and    -   their turn-on time period t_(on) is chosen so that their next        turn-on time (at which the current returns to zero) is as        required in the post-transition period.

The new phase circuit 20 being activated is inserted to be last in thepost-transition phase, after the last of the Slave phases and before theMaster phase. Its next turn-on time 31′ is as required in thepost-transition period. Its only free parameter is its turn-on timeperiod t_(on): The turn-on time period t_(on) determines its periodlength T which in turn, going backwards in time from the next turn-ontime 31′, determines the first turn-on time 31 when activating the newphase circuit 20. The turn-on time period t_(on) is chosen such that thetotal current of all phase circuits 20 is minimises its deviation fromthe total current set point.

FIG. 8 shows deactivation of a phase circuit and removal of its currentpulses from a sequence of interleaved pulses. Given a total current setpoint, in a pre-transition period the current pulses are lower and theperiod length Tpd is shorter than in a post-transition period withperiod length Tsd. A transition period has the same period length Tpd asthe pre-transition period and coincides with the pulse period of a phasethat is designated as the Master. The other phases (Slaves) except theone following the Master are operated to be

-   -   switched on in the transition period, with turn-on times        (relative to the period) as in the pre-transition period, and    -   their turn-on time period t_(on) is chosen so that their next        turn-on time (at which the current returns to zero) is as        required in the post-transition period.

The phase circuit 20 following the Master is also, like the otherSlaves, switched on with a turn-on time 31 as in the pre-transitionperiod. Here too, its only free parameter for shaping the last pulse isits turn-on time period t_(on): The turn-on time period t_(on)determines the period length T and the point at which its currentreturns to zero, ending the last pulse. The turn-on time period t_(on)is chosen such that the total current of all phase circuits 20 isminimises its deviation from the total current set point.

FIG. 9 shows switching between DCM and BCM and vice versa: the upperdiagram shoes four phase circuits A, B, C, D operating in DCM andpairwise. In each pair (A-C and B-D), the two phase circuits generatealternating pulses, with one phase circuit (A or B) generating a currentpulse while the other one (C or D) provides no current, and vice versa.The pulses from the two pairs are interleaved. Switching to BCM can bedone individually in each pair, by deactivating one of the two phasecircuits (C and D) and operating the other one (A and B) to generateboth pulses in BCM. The pattern of the total current remains the same.

Switching from BCM to DCM can be done in an analogous way, i.e., byswitching one phase circuit 20 from BCM to DCM and thereby omittingalternating pulses, and activating another phase circuit 20 to supplythe omitted pulses.

More generally (not illustrated), in the same way an integer multiple ofphases operating in DCM to generate a sequence of adjacent pulses can bereplaced by a single phase operating in BCM, and vice versa.

Such a switch from BCM to DCM or vice versa can be applied in conditionswhere the load, or required total current requires it. For example, whena relatively small current is required, the maximum frequency or minimumturn-on time period ton may not allow for BCM, and thus DCM must beused.

FIG. 10 shows a method for controlling the multiphase DC-DC converter 10in DCM. Its purpose is to trigger pulses from one or more phase circuits20 operating in DCM, given a total current set point to be delivered bythe converter. A sequence of phase current pulses of essentially thesame shape is generated in the following way:

-   -   a peak current I_(Peak) and period length T, defining the shape        of the pulses, is determined;    -   an integrator is initialised with a value corresponding to the        product of the peak current I_(Peak) and the period length T;    -   a current pulse with the peak current I_(Peak) and period length        T is triggered in one of the phase circuits 20;    -   the integrator integrates the total current set point value 41,        with its integrator output 42 running towards zero;    -   when the integrator output 42 reaches zero, a next current pulse        in another phase circuit 20 is triggered by a trigger pulse 43,        the integrator is initialised again, and the procedure continues        with the preceding step.

Limits for ranges in which the peak current I_(Peak) and period length Tcan be chosen depend on the hardware and operation considerations.Relevant parameters for the choice can be a maximum switching frequency,minimal pulse length, switching losses.

The trigger pulses 43 are multiplexed over the active phase circuits 20.The sum of the current pulses will correspond to the total current setpoint 41. Thanks to the simple structure, the total current set point 41can be tracked with low latency.

The example in FIG. 10 shows the integrator being initialised to anegative value and then integrating in the positive direction. It isunderstood that the same principle can be realised with inverted signsand with differently scaled values of the signals involved.

In typical applications, the following values can be present for the

-   -   period length T: corresponding to a frequency of 10 kHz to 700        kHz, in particular of 20 kHz to 400 kHz.    -   number of phases N: 6 to 12 phases.    -   peak current per phase: up to 120 A or up to 200 A or more.    -   mean current per phase: up to 60 A or up to 100 A or more.    -   total current: up to 600 A, up to 800 A or more.    -   output voltage: 200 V to 800 V.    -   inductance value of the inductor 21: 5 to 20 micro-H or 8 to 15        micro-H,

While the invention has been described in present embodiments, it isdistinctly understood that the invention is not limited thereto, but maybe otherwise variously embodied and practised within the scope of theclaims.

1. A control method for controlling a multiphase DC-DC converter themultiphase DC-DC converter being arranged for exchanging electricalpower between an input side an output side, the multiphase DC-DCconverter comprising two or more phase circuits, each phase circuitcomprising a switch and an inductor, the switch being arranged tocontrol an inductor current I_(L) through the inductor, the phasecircuit being arranged to generate a phase current contributing to atotal current to be delivered to an output side of the multiphase DC-DCconverter, the method comprising: switching two or more of the phasecircuits in Boundary Conduction Mode (BCM) to generate interleaved phasecurrent pulses, with a period length T and a nominal turn-on time periodt_(on) of the switch; in at least one of the two or more of the phasecircuits being switched, and for successive phases, repeatedly adaptingthe turn-on time period t_(on) for controlling the length of the pulsesof the inductor current I_(L) to minimise a difference from the periodlength T.
 2. The control method of claim 1, the multiphase DC-DCconverter being arranged for exchanging electrical power between aninput side, comprising a first input terminal and a second inputterminal, and an output side, comprising a first output terminal and asecond output terminal, the multiphase DC-DC converter comprising two ormore phase circuits, each phase circuit comprising an inductor connectedbetween the first input terminal and a bridge point, an upper branchswitching unit connected between the bridge point and the first outputterminal, and a switch with a parallel freewheeling diode connectedbetween the bridge point and the second input terminal and second outputterminal, the control method comprising the steps of, determining anumber N of phase circuits to be active; determining an initial valuefor a turn-on time period t_(on); determining a period length T; for atleast one of the phase circuits that are to be active; turning on theswitch of the phase circuit, causing an inductor current I_(L) and theswitch to increase over time, and determining a target turn-on time whenthe switch shall be turned on the next time; after the turn-on timeperiod t_(on), switching off the switch, causing the inductor currentI_(L) to flow through the upper branch switching unit and decrease overtime; after the current through the inductor has returned to zero, at azero crossing time, turning the switch on again and repeating the abovesteps; when repeating the above steps, controlling the turn-on timeperiod t_(on) by increasing the turn-on time period t_(on) if the zerocrossing time is before the target turn-on time and by decreasing theturn-on time period t_(on) if the zero crossing time is after the targetturn-on time.
 3. The control method of 1, wherein the initial value forthe turn-on time period t_(on) of a phase circuit is computed as:$t_{on} = \frac{2 \cdot I_{mean} \cdot L}{U_{IN}}$ wherein U_(IN) is thevoltage at the input side, L is the inductance value of the inductor andI_(mean) is the mean current to be delivered by the phase circuit. 4.The control method of claim 1, wherein in the active phase circuits, therespective period length T is computed as$T = {t_{on}\frac{U_{OUT}}{U_{OUT} - U_{IN}}}$ wherein U_(OUT) is thevoltage at the output side.
 5. The control method of claim 2, whereinthe step of determining the target turn-on time comprises computing thetarget turn-on time as being offset from a reference time by the periodlength T divided by the number N of phase circuits that are active. 6.The control method of claim 2, wherein the step of turning the switch onagain comprises one of: monitoring the voltage across the switch andturning on the switch when the voltage across the switch is zero;monitoring the voltage across the switch, and turning on the switch apredetermined time delay after the voltage across the switch falls undera predetermined threshold; monitoring the inductor current I_(L) andturning on the switch when the inductor current I_(L) becomes zero afterhaving been negative due to a reverse current through the upper branchswitching unit; monitoring the current through the upper branchswitching unit, in particular a diode current, and turning on the switchwhen this current becomes zero after having been negative due to areverse current through the upper branch switching unit.
 7. The controlmethod of claim 2, wherein the step of turning the switch on againcomprises: when the inductor current I_(L) has returned to zero, lettingthe current reverse its direction and continue to flow through theinductor and the upper branch switching unit until the upper branchswitching unit is turned off and the inductor current I_(L), commutatesto the freewheeling diode; turning on the switch; in particular whereinthe upper branch switching unit is constituted by a diode, or comprisesa diode, and is turned off by a reverse charge in the diode having builtup, and in wherein a reverse charge of the diode is chosen so that thereverse current through the diode is sufficient to dischargecapacitances between the bridge point and the second input terminal. 8.The method of claim 1, further comprising, for increasing the number Nof phase circuits that are active to N+1, given a total current setpoint, in a pre-transition period of length Tpi, switching the switchesof the N phase circuits to turn on at turn-on times, relative to thisperiod, of 0, dTpi, 2·dTpi, 3·dTpi . . . (N−1)·dTpi where dTpi=Tpi/N;computing, for a post-transition period of length Tsi, target turn-ontimes for the N+1 phase circuits, relative to this period, as 0, dTsi,2·dTsi, 3·dTsi . . . N·dTsi where dTsi=Tsi/(N+1); in a transitionperiod, switching the switches of the N phase circuits to turn on at thesame turn-on times, relative to this period, as in the pre-transitionperiod; in the transition period, for each of the N phase circuits,setting the turn-on time periods t_(on) so that the current returns tozero at the respective target turn-on time in the post-transitionperiod; in the transition period, after the turn-on time at (N−1)·dTpi,turn on the switch of the newly operated (N+1)th at a time and with aturn-on time period t_(on) such that a deviation of the total current ofall phase circuits in the transition period and the post-transitionperiod from the total current set point is minimised.
 9. The method ofclaim 1, further comprising, for decreasing the number N of phasecircuits that are active to N−1, given a total current set point, in apre-transition period of length Tpd, switching the switches of the Nphase circuits to turn on at turn-on times, relative to this period, of0, dTpd, 2·dTpd, 3·dTpd . . . (N−1)·dTpd where dTpd=Tpd/N; computing,for a post-transition period of length Tsd, target turn-on times for theN−1 phase circuits, relative to this period, as 0, dTsd, 2·dTsd, 3·dTsd. . . (N−2)·dTsd where dTsd=Tsd/(N−1); in a transition period, switchingthe switches of the N phase circuits to turn on at the same turn-ontimes, relative to this period, as in the pre-transition period; in thetransition period, for each of the N phase circuits, except for thephase circuit whose target turn on time relative to this period is dTpd,setting the turn-on time periods t_(on) so that the current returns tozero at the respective target turn-on time in the post-transitionperiod; in the transition period, for the phase circuit whose targetturn on time relative to this period is dTpd, setting the turn-on timeperiod t_(on) for one last pulse of this phase circuit such that adeviation of the total current of all phase circuits in the transitionperiod and the post-transition period from the total current set pointis minimised.
 10. The method of claim 1, further comprising, for atransition between operation of the multiphase DC-DC converter indiscontinuous conduction mode to boundary conduction mode, for one ormore sets of np phase circuits each, np being two or more, for each ofsaid sets: operating the respective np phase circuits of the set indiscontinuous conduction mode to generate a sequence of np pairwiseadjacent current pulses of period length T, each phase circuitcontributing one of the current pulses of the sequence; not operatingthe respective np phase circuits of the set in discontinuous conductionmode, and instead operating one phase circuit in boundary conductionmode to continue the sequence of current pulses of period length T. 11.The method of claim 1, further comprising, for a transition betweenoperation of the multiphase DC-DC converter in boundary conduction modeto discontinuous conduction mode, for one or more sets of np phasecircuits each, np being two or more, for each of said sets: operatingone phase circuit in boundary conduction mode to generate a sequence ofcurrent pulses with period length T in boundary conduction mode; notoperating this one phase circuit in boundary conduction mode, andinstead operating the respective np phase circuits of the set tocontinue the sequence of current pulses of period length T by operatingthe np phase circuits in discontinuous conduction mode to generate asequence of np pairwise adjacent current pulses of period length T, eachphase circuit contributing one of the current pulses of the sequence.12. The method of claim 1, further comprising determining the periodlength T by operating one of the phase circuits, called master phase,with a turn-on time period t_(on) that is determined according to a meancurrent to be delivered by this phase circuit, and operating one or moreof the remaining active phase circuits to adapt their timing and periodlength to that of the master phase.
 13. A multiphase DC-DC converter,comprising a controller comprising voltage sensors arranged fordetermining the voltage U_(IN) at the input side, the voltage U_(OUT) atthe output side and the voltage U_(L) across the inductor of each phasecircuit, the controller being configured to perform the method ofclaim
 1. 14. The multiphase DC-DC converter of claim 13, wherein in atleast one of the phase circuits the upper branch switching unitcomprises or consists of a diode that comprises a reverse recovery timethat is sufficiently large to reverse the inductor current I_(L) afterthe inductor current I_(L) has returned to zero, such that the reversedcurrent discharges a capacitance of the switch and of a freewheelingdiode and of a parallel capacitance, if present, before turning on theswitch.
 15. The multiphase DC-DC circuit of claim 14, wherein thereverse recovery time that is sufficiently large for the reversedcurrent to also discharge a capacitor arranged parallel to the switchbefore turning on the switch.